High voltages can be applied and high currents can be conducted in some power semiconductor switches. Short circuits or overcurrents can quickly lead to thermal destruction of the power semiconductor switches. Power semiconductor switches can include protective circuits for the detection of short circuits and overcurrent situations. One way of detecting these situations is by indirectly monitoring the current through the power semiconductor switch by determining the voltage across the power semiconductor switch. The voltage across the power semiconductor switch should fall swiftly from a relatively high level in the switched-off condition (the “switched-off-condition” or “OFF-condition” is a condition of the power semiconductor switch in which it is “open” and does not conduct current), to a relatively low level in the switched-on condition (the “switched-on-condition” or “ON-condition” is a condition of the power semiconductor switch in which it is “closed” and may conduct current). Accordingly, a control signal of a power semiconductor switch (for example a gate-driving signal) has an ON-condition, in which it keeps the power semiconductor switch closed and an OFF-condition, in which it keeps the power semiconductor switch open.
The curve on the lower left in FIG. 1B shows an exemplary change in the collector-emitter voltage of an IGBT (an IGBT is an exemplary power semiconductor switch) after a transition from a switched-off condition to a switched-on condition (the change in a corresponding exemplary control signal 130 is shown above left) in normal operation. As shown, the collector-emitter voltage drops sharply to a very low value (close to 0 volts, for example between 0 and 10 V). An exemplary short circuit behavior of an IGBT is shown below right in FIG. 1B. In contrast to normal operation, the collector-emitter voltage does not fall to the very low value, but high currents can flow in the IGBT (for example between three and ten times the nominal current of the IGBT). In other short circuit cases, the collector-emitter voltage first drops to its value under normal operation but rises again after some time. This may result in a high thermal loading of the power semiconductor switch which can be damaged in a relatively short time. Some IGBTs for example, can withstand a short circuit for some time (e.g., 10 μs) in the switched-on condition without damage. Protective circuits for the detection of a short circuit or overcurrent situation should turn off the power semiconductor switch before the end of this time period. Similar characteristics can also be found in other power semiconductor switches other than in IGBTs. An overcurrent state may as a short circuit state lead to an increased collector-emitter voltage. The collector-emitter voltage in an overcurrent state may be closer to the collector-emitter voltage in the normal state than in the short-circuit state.
An overcurrent or short-circuit state can also occur a period of time after the power switch has been turned on. In this error case, the collector-emitter voltage can drop to a low value associated with normal operation before an error occurs (e.g., leading to a short-circuit). This can result in a sharp increase in the collector-emitter-voltage (not shown in FIG. 1B), and can also have the negative effects on the power semiconductor switch described above.